1. Field of the Invention
The present invention relates to a polishing system which automatically transfers in, polishes, and transfers out magnetic disks or other workpieces.
2. Description of the Related Art
As this type of polishing system, there is, for example, the related art shown in FIG. 10.
In FIG. 10, reference numeral 110 is a first transfer unit for transferring the unpolished workpieces W in, reference numeral 120 is a polishing unit for polishing the transferred workpieces W to a predetermined thickness, and reference numeral 130 is a second transfer unit for transferring the workpieces W polished at the polishing unit 120 out.
The first transfer unit 110 picks up workpieces W arranged on a table 111 by chucks 113 fixed to a loader 112, moves the loader 112 along a rail 140 then transfers the workpieces to holding holes 121a of carriers 121 of the polishing unit 120.
The polishing unit 120 grips the workpieces W held in the carriers 121 between a lower plate 122 and an upper plate 123, drives a sun gear 124 to rotate, and makes the upper and lower platens 122 and 123 rotate in mutually opposite directions so as to polish the two surfaces of the workpieces W.
That is, by driving the sun gear 124 to rotate, the carriers 121 rotate and revolve in the donut-shaped shape between the sun gear 124 and an internal gear 125, so the workpieces W in the carriers 121 are effectively polished by the oppositely rotating lower platen 122 and upper platen 123. The polishing work is stopped when the workpieces W are polished to the desired thickness.
The second transfer unit 130 moves an unloader 131 along the rail 140 to the polishing unit 120 side, takes out the polished workpieces W from the holding holes 121a of the carriers 121 by chucks 132 fixed to the unloader 131, then carries the workpieces to a table 133 and arranges them there.
In order for the loader 112 of the first transfer unit 110 to reliably transfer the unpolished workpieces W into the holding holes 121a of the carriers 121 and for the unloader 131 of the second transfer unit 130 to reliably transfer the polished workpieces W from the holding holes 121 a of the carriers 121, however, it is necessary that the carriers 121 and the holding holes 121a be returned to the positions at the time of start of the polishing when the polishing is finished.
FIGS. 11A and 11B are schematic plan views for explaining the positions of the carriers and holding holes. FIG. 11A shows the positions at the time of start of the polishing, while FIG. 11B shows the positions at the end of the polishing.
As shown in FIG. 11A, if the polishing is started when the centers of the three carriers 121-1 to 121-3 and the centers of the holding holes 121a lie on three straight lines m 120 degrees apart, when the polishing is ended, if the centers of the three carriers 121-1 to 121-3 are off from the lines m as shown by the broken lines in FIG. 11B or if the centers of the carriers 121-1 to 121-3 are aligned with the lines m but the centers of the holding holes 121a are not on the lines m as shown by the solid lines, the chucks 113 and 132 fixed to the loader 112 and unloader 131 will not be able to transfer the workpieces W into and out from the holding holes 121a of the carriers 121.
Accordingly, when ending the polishing, it is also necessary that the centers of the carriers 121 and the centers of the holding holes 121a be positioned on the lines m.
Therefore, this polishing system is designed to set the gear ratio among the sun gear 124, the internal gear 125, and the carriers 121 to 1:3:1 so that when the sun gear 124 rotates by exactly a whole multiple of 4, the carriers 121 will return to their positions at the start of revolution and the holding holes 121a of the carriers 121 will return to their positions at the start of their rotation.
That is, the system is designed so that when the sun gear 124 rotates by exactly a whole multiple of 4 from the state shown in FIG. 11A, the system will return to the start where the centers of the carriers 121-1 to 121-3 lie on the three straight lines m and the holding holes 121a of the carriers 121 lie on the lines m (hereinafter referred to as the "predetermined position state"). This makes it possible for the workpieces W to be reliably transferred into or transferred out of the holding holes 121a of the carriers 121 by the loader 112 or the unloader 131.
In the above polishing system of the related art, however, there were the following problems.
For example, when polishing workpieces W from the state shown in FIG. 11A and stopping the drive of the polishing unit 120 when the workpieces W are polished to a desired thickness, the carriers 121 and holding holes 121a will almost never end up in the above predetermined position state in practice.
Therefore, in the polishing system of the related art, it was necessary to rotate the sun gear 124 several times after the workpieces W were polished to the desired thickness so as to return the carriers 121 and the holding holes 121a to the predetermined position state.
Even if the sun gear 124 is rotated just once, however, the carriers 121 will rotate and revolve a considerable number of times. During that time, the workpieces W will end up being polished more than necessary.
In particular, when the carriers 121 or the holding holes 121a are slightly off from the predetermined position state, despite the workpieces W being the desired thickness, the sun gear 124 must be rotated as much as about 4 times in order to return the carriers 121 and the holding holes 121a to the predetermined position state. The loss in the workpieces W ends up becoming remarkably great in such a case.
In this way, in the polishing system of the related art, it was necessary to sacrifice some of the polishing precision of the workpieces W in order to reliably transfer the workpieces W in and out. In particular, this problem was conspicuous with a polishing unit 120 with a high polishing rate.